1. Field of Invention
The present invention relates to an apparatus that permits loads requiring low nominal voltages (such as automobile lights) and loads requiring high voltages (such as air conditioning units, power steering systems, rear-window defoggers, and valve trains) to co-exist in an automobile electronics system using a simple and inexpensive arrangement to supply the operating voltages.
2. Background of Related Art
New automobile systems commonly include both a high voltage source and a low voltage source. A typical nominal value for the high voltage source is 42V, while a typical nominal value for the low voltage source is 14V. The high voltage source is included to power those automobile components that require larger amounts of energy and to accommodate new features in automotive systems and controls. The low voltage source is also maintained in order to power conventional automobile devices which cannot handle the peak voltages and fluctuations that accompany the larger voltage source. For instance, the wire and filaments for conventional automobile lamps would burn out rapidly, sometimes explosively, if driven by a higher 42V battery. Such effects create unwanted system failures and safety concerns. Replacing the conventional low voltage lamp with a high voltage lamp is not presently a practical solution in view of the widespread use of the low voltage lamps throughout the automotive industry. Moreover, higher voltage lamps are characterized by longer and thinner filaments than those present in their low voltage counterparts. This makes high voltage lamps more fragile and susceptible to both mechanical and electrical damage and thus less suitable for automotive applications. Therefore, higher voltage automobile power systems must be able to supply both high and low voltage loads.
In order to permit both the high voltage and low voltage sources to co-exist, conventional devices require the use of a high capacity DC/DC converter as shown in FIG. 1. Referring to FIG. 1, an alternator/generator A charges a 42V battery. In order to convert the 42 volt supply voltage to a lower voltage, a DC/DC converter CV is connected to the positive terminals of the low and high voltage batteries in a common ground system (other configurations are also possible). A lamp L2 is switched on and off by a switch SW on the low voltage side of the circuit. As illustrated in FIG. 1, the assignee of the present application has also been able to operate some 12 V lamps L1 on a high voltage side of the circuit via a pulse width modulation circuit PWM. The high voltage side also includes a 42 V load operated by a switch. The high voltage load may comprise any form of resistive, reactive and inductive elements. The nature of this load does not significantly impact the conventional system or the invention. On the high voltage side of the system, since the lamps L1 draw a significant amount of the 42V from the high voltage source, the flickering and burn-out of these lamps may result. The difficulty of providing PWM control for many low voltage loads cannot sufficiently eliminate these effects. Moreover, the presence of the conventional DC/DC converter adds much to the weight and complexity of an automobile power distribution system. The converter is also a bulky piece of apparatus that detracts from the space available in the passenger cabin or otherwise adds to the size of the vehicle. Manufacturing costs also increase with the inclusion of a DC/DC converter. It is estimated that a DC/DC converter can cost as much as $1.00 per watt of conversion capacity. Currently, a typical minimum conversion requirement for automotive applications is on the order of 1000 Watts (1 kW), which adds significantly to vehicle design and manufacturing costs.
In lieu of the DC/DC converter, the system may alternatively include a separate alternator/generator for the low voltage supply source. However, this arrangement also includes many of the deficiencies of the DC/DC converter system in terms of space, weight, cost and complexity.
Applicant""s invention overcomes the above deficiencies by eliminating or reducing the requirements for a complex and bulky DC/DC converter. Rather than having lamps directly driven by the high voltage source and having a low voltage battery charged by a DC/DC converter, the present invention alters the terminal connections between the high voltage battery, the low voltage battery, a pulse width modulation circuit and a load, which draws current. By connecting the load through a pulse width modulation circuit to a positive terminal of both the high voltage battery and also to the positive terminal of the low voltage battery, the voltage drop across lamps connected to the high voltage side of the battery can be reduced by the nominal amount of the low battery voltage. A typical value is 14 volts. Therefore, rather than drawing all 42V of a high voltage power source, the voltage drop across the lamps is reduced by 14V, and the 28 volts drawn by the lamps can be more easily managed using pulse width modulation techniques. Adequate charging of the high voltage source is provided by selection of an appropriate alternator.
Additionally, the manually operated load is connected in parallel via a switch with the 14V battery, permitting the battery to be charged even without the inclusion of a DC/DC converter. Should a small step up in energy in the 14V-system be necessary, this can be achieved with a more simplified arrangement and can also be achieved by running the PWM-operated lamps (loads) at below a luminance (operating) value. A similar effect can also be accomplished by creating a pseudo switch mode converter using the inductance of the connected inductive loads such as motors and or their resistance.